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Zika virus

Abstract

This mosquito borne virus is in the news , because of its ability to cause microcephaly in babies and the potential risks to spectators and athletes at the Rio Olympic Games .

Overview.

Discovered in Uganda in 1947, the Zika virus has long been thought to be a virus of little importance to humans from a health perspective. This view very quickly shifted in 2015 as the Zika virus hit Brazil and the Americas. Along with the outbreak of the Zika virus there has been accompanying fear and misinformation. This overview seeks to provide a balanced appraisal of what we know about Zika virus and address key questions relating to it.

What is the Zika virus?

The Zika virus is a Flavivirus that is predominately a mosquito borne virus. It is transferred principally by a specific species of mosquito known as Aedes aegypti. This is the same mosquito species known to carry other viruses such as the Dengue fever and Yellow fever virus. There are some other mosquito species implicated in the transmission of the Zika virus, though they are implicated relatively infrequently.

Can the Zika virus be spread in ways other than mosquito bites?

Yes. There has been considerable mounting evidence that the Zika virus can also be sexually transmitted (1). There is a potential risk of transmission of the Zika virus through blood transfusion. As of yet there are no known cases of Zika virus being transmitted though blood transfusion though (2).

What diseases is Zika known to cause?

An initial flu-like illness:

Initial infection with the Zika virus is known to cause a mild acute febrile illness. Symptoms are typical of many viral illnesses and include fever, rash, joint aches (arthralgia), muscle aches (myalgia), headache, viral conjunctivitis, and vomiting (2). These symptoms are thought to occur within roughly 1 week of contracting the virus (2). Overall the symptoms are fairly non-specific and hard to differentiate from other viral illnesses.

Neurological complications:

Some observational studies looking at health outcomes in areas where there have been Zika virus outbreaks e.g. the Pacific and the Americas have demonstrated an increased occurrence of a demyelinating neuropathy known as Guillain–Barré syndrome (3). Though studies have shown a strong association between Zika virus infection and Guillain–Barré syndrome (4) as of yet a causative link has not yet been fully established (3).

Guillain–Barré syndrome is a syndrome caused by the body’s immune system attacking nerves in the body. It can be triggered by a number of bacterial or viral infections. The resultant nerve damage leads to muscle weakness, loss of sensation, and can be a cause of severe disability and death.

Birth defects:

The most worrisome and well-publicized effect of Zika virus infection is that of microcephaly. There is substantial evidence existing indicating that the Zika virus is transmitted from mother to fetus during pregnancy.

This link was first noticed in September 2015 in Brazil where the recognition of the Zika virus outbreak coincided with an increase in the incidence of microcephaly (5). Since then a causal relationship between prenatal Zika virus infection and microcephaly has been demonstrated (5).

The frequency and risk factors of this vertical transmission are unknown as of yet (2). The greatest risk of Zika virus infection is likely during the first trimester though it is thought damage can be caused as late as early in the third trimester (2). Not all women with prenatal Zika virus infection will give birth to babies with microcephaly. It isn’t currently clear why some mothers infected with the Zika virus will give birth to a baby with microcephaly whilst others do not.

Microcephaly is a clinical finding where the head is smaller than expected for gestational age and sex. It is indicative of abnormal brain development. It should be noted that there are a multitude of causes of microcephaly other than pre-natal Zika virus infection. In essence anything that disrupts brain development can cause microcephaly. Microcephaly, irrespective of cause, is accompanied by a generally poor life trajectory for sufferers involving varying degrees of intellectual disability, neurologic defects, and reduced life expectancy. There is no treatment existing that can reverse the abnormal brain development.

Who is at risk of contracting and being affected by the Zika Virus?

The Centers for Disease Control and Prevention monitors and lists those countries with known active Zika virus transmission. This information can be found at www.cdc.gov/zika/geo/active-countries.html. Essentially anyone living in or travelling to these countries is at risk of being infected with the Zika virus.

On the basis of the known link between Zika virus infection and microcephaly there is little doubt that women who are currently pregnant, or who are trying to get pregnant, have the greatest potential burden of disease. For this reason they are advised not to travel to countries with active Zika virus transmission.

What measures can be undertaken to protect against the Zika virus?

There are a number of measures that can be taken to protect against infection with the Zika virus. The majority of these revolve around reducing the risk of being bitten by an Aedes aegypti mosquito. The Aedes aegypti mosquito predominately bites during daylight hours. Hence it is during the day that the greatest amount of vigilance needs to be employed. Some key protective measures include:

Avoid unnecessary travel to areas where there is ongoing Zika virus transmission. This is particularly important for pregnant women or women trying to get pregnant.

Avoid unprotected sexual intercourse with people with a recent travel history to areas of ongoing Zika virus transmission. Again, this is particularly important for pregnant women or women trying to get pregnant.

The use of physical barriers such as nets, window screens, and closing windows and doors as able. To this end staying in air-conditioned accommodation can be very helpful.

Avoid travel to areas with poor sanitation and no piped water. Aedes aegypti mosquitoes require still water to breed and therefore the risk of being bitten in these areas is thought to be higher.

What about the Olympics being in Rio de Janeiro this year?

The current position the World Health Organization has taken is that “cancelling or changing the location of the 2016 Olympics will not significantly alter the international spread of Zika virus” (7).

For anyone considering travel to Rio de Janerio for the Olympics preventative measures as outlined should be employed. The World Health Organization recommends that pregnant women do not travel to Brazil, as it is an area of ongoing Zika virus transmission.

How do I know if I have contracted the Zika virus?

The acute illness caused by Zika virus infection is relatively non-specific, being similar to many other viral illnesses. The key factor then that should lead to suspicion of Zika virus infection is recent travel to an area where Zika virus is known to be present, coupled with onset of a flu-like illness previously described.

In instances such as these where Zika virus infection is suspected there are a number of tests that can be undertaken by a doctor to confirm or exclude the diagnosis. The tests involve detecting the presence of Zika virus RNA (genetic material) in bodily fluids (blood or saliva, or detecting antibodies against the Zika virus in the blood.

RT-PCR, the test used to detect Zika virus RNA, is expected to be positive within 1 week of the onset of clinical illness, as this is when the virus is present in the blood (2). Detection of antibodies (IgM) against the Zika virus will likely be positive after the initial illness has subsided and for several months following this (2). As the testing for the Zika virus is a relatively recent undertaking there is still a lot that is not known in relation to the subtleties of these tests in relation to diagnosis.

Testing the fetus prenatally for evidence of Zika virus infection is also not well established. Amniotic fluid can be tested for evidence of Zika virus RNA (genetic material.) Cord blood can also be tested at the time of birth (2). Ultrasonography can be used to detect microcephaly prenatally however this is not always reliable.

Are there any treatment or vaccines available against the Zika virus?

No. There currently doesn’t exist any treatments or vaccines against the Zika virus. Prevention measures against contracting the Zika virus are therefore all the more important.

What do we still not know about the Zika virus and its health ramifications?

A lot.

It isn’t clear why some mothers infected with the Zika virus will give birth to a baby with microcephaly whilst others do not. It isn’t known if there is a specific point during pregnancy where the risk of infection is greatest. It isn’t clear exactly why or how the Zika virus halts brain development and causes microcephaly. It isn’t clear how wide the spectrum of birth defects caused by prenatal Zika virus infection is.

There is also some worry amongst scientists, clinicians, public health officials, and governments that the Zika virus has a role in the development of other neurological problems. It may be the case that we’re only scratching the surface in regards to the true burden of disease accounted for by the Zika virus.

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